A variety of commercial processes rely on the use of fluid separation techniques in order to separate one or more desirable fluid components from a mixture. For example, in the production of natural gas, it is typically necessary for the producer to strip carbon dioxide from natural gas in order to meet government regulatory requirements. It is also typically desirable in many chemical processes for hydrogen to be removed and recovered from gaseous process streams.
The use of membranes for fluid separations has achieved increased popularity over other known separation techniques. However, one major disadvantage of membranes for use in fluid separations is that the membranes must be supported in a "package" (sometimes referred to as a "module") which provides the requisite flow path to achieve the desired fluid separation. The membrane package must also exhibit sufficient structural integrity to withstand the pressures needed to effect separation in a given process. These physical demands of membrane packages become especially acute when the membrane package is used in high pressure separation processes (i.e., fluid separation processes having operating pressures of greater than about 500-1000 psi).
Recently, a stacked membrane disk assembly for fluid separations has been proposed in U.S. Pat. No. 4,613,436 issued to William W. Wight et al on Sep. 23, 1986 (hereinafter more simply referenced as "the Wight et al '436 Patent", the entire content of which is expressly incorporated hereinto by reference). According to the Wight et al '436 Patent, a compact stack of alternating layers of membrane disks with layers of feed fluid spacers is suggested. Each such layer is provided with a pair of notches formed in the perimetrical edge and a central aperture. The respective notches and apertures of each layer are registered when stacked such that the registered notches collectively form respective feed and residue channels, while the registered apertures collectively form a permeate channel.
Sealing beads (e.g., bead 40 shown in FIG. 3 and bead 57 shown in FIG. 4 of the Wight et al '436 Patent) extend around the perimetrical edge of the feed fluid spacers from one notch to the other. The sealing beads are thus discontinuous around the edge of the feed fluid spacers in the area of the notches (see, column 9, lines 18-21 of the Wight et al '436 Patent). In use, these discontinuous edge beads cooperate in conjunction with edge seals associated with the distribution plates and feed fluid spacers of the assembly disclosed in the Wight et al '436 Patent (i.e., edge seals 104 and 180 shown in FIGS. 9 and 10, respectively). These edge seals thereby serve as a pressure seal against the interior surface of the pressure vessel in which the assembly is positioned so that the feed and residue channels are collectively defined by the registered notches and a corresponding adjacent interior surface of the pressure vessel wall. In addition, the discontinuous sealing beads employed in the assembly disclosed in the Wight et al '436 Patent serve to fluid-isolate interior regions of the pressure vessel on opposing sides of the stacked membrane disk assembly so as to establish a pressure gradient (e.g., between 1-5 psi) between the feed inlet and residue discharge ports.
While the stacked membrane disk assembly disclosed in the Wight et al '436 Patent represented a significant advance in the art of fluid separations, there exist several practical disadvantages associated with the assembly's required notches and discontinuous sealing beads. As noted above, since the sealing beads are discontinuous, in order to effectively fluid-isolate the feed and residue channels from one another, the beads must make effective sealing contact with the interior surface of the pressure vessel wall. As a consequence, the interior surface of the pressure vessel wall must be machined to very high tolerances to prevent even the slightest gap from existing between the discontinuous sealing beads and the interior surface vessel wall. For example, at high operating pressures of greater than between about 500-1000 psi, a very small gap between the discontinuous sealing beads and the interior surface vessel wall could cause failure of the membrane assembly (i.e., prevent the membrane assembly from performing its intended fluid separation functions).
Improvements to the stacked membrane disk assembly disclosed in the Wight et al '436 Patent are proposed in the above-referenced copending and commonly owned U.S. patent application Ser. No. 08/241,371. In this regard, a principal improvement disclosed in that copending Patent Application is embodied in a self-contained stacked membrane assembly. That is, the membrane module assembly disclosed in that copending Patent Application is itself pressure-isolated and thus does not require edge seals to be formed with the interior surface of the pressure vessel wall. As a result, the only requirement on the internal dimensions of the pressure vessel is that sufficient space be provided to house the membrane assembly. In other words, the longitudinal and/or latitudinal size of the pressure vessel wall can be virtually any dimension since the perimetrical edge of the membrane assembly does not need to be sealed against the interior pressure vessel wall in order to perform its intended fluid-separation functions.
In use, membrane module assemblies are subjected to repeated pressurization and depressurization cycles due to periodic equipment maintenance and day-to-day processing needs requiring equipment shut-down. When stacked membrane disk modules of the type disclosed in the copending Patent Application cited above are pressurized, stone of the structural components may undergo irreversible dimensional deformation. For example, the membrane and/or permeate carrier sheets in the membrane disks may be dimensionally "thinned" when subjected to relatively high pressures (sometimes on the order of greater than 1000 psi) of a given fluid separation process. At the same time, however, the seals employed will typically be sufficiently elastic and/or compressible so that they will maintain a fluid and pressure tight seal with the membrane disks when pressurized.
However, the seals in the module may not be capable of elastically responding to a rapidly decreasing pressure when the module is depressurized sufficient to maintain a reliable seal with the membrane disks. As a result, it is likely that the seals will be displaced and/or lose sealing contact with the membrane disk during depressurization which, in turn, causes fluid leakage to occur if the module is subsequently attempted to be repressurized. Such fluid leakage will thus cause the module to fail to preform its intended fluid-separation functions. Once fluid leakage has occurred, therefore, the only alternative is to remove the module from service and rebuild the membrane disk and seal components.
It would therefore be desirable if means were provided which would allow a stacked membrane disk assembly to undergo repeated pressurization/depressurization cycles without failure (e.g., without experiencing fluid leakage on depressurization). According to the invention disclosed and claimed in the above-cited copending U.S. patent application Ser. No. 08/240,509, such means are provided in the form of an assembly having a perforated permeate tube and biasing structures.
It would, however, be especially desirable if the permeate tube and the biasing structure were provided as an integral unit (e.g., so as to promote ease of assembly and to reduce module space). It is towards fulfilling such a need that the present invention is directed.